Radioactive wastes arise in a variety of chemical and physical forms at every stage of the nuclear fuel cycle. Some of the radioisotopes in these wastes are so long-lived that they must be isolated from the biosphere for many thousands of years. Although much work has been done over the last 30 years to develop techniques for separating and immobilizing these materials, much work remains to be done.
A significant problem has occurred in attempting to separate and immobilize radioactive materials (e.g., radioisotopes) from acidic waste streams that are generated by most if not all nuclear facilities. The current practice in treating such waste streams requires caustic precipitation of insoluble metal oxides and hydroxides followed by the removal of the base soluble metal ions with a zeolite ion exchange material (e.g., chabazite or Zeolon-900). The sludge which precipitates when the acidic stream is neutralized requires further treatment. The volume is reduced by evaporation and the solids are processed into a non-leachable form for storage or disposal. It would be advantageous to provide a simplified process that avoided the necessity of precipitating and treating such a sludge.
Acid stable ion exchange resins in place of the zeolite ion exchange material would allow removal of the radioactive waste without requiring caustic precipitation and subsequent sludge treatment. Acid stable ion exchange resins cannot, however, be used because of their limited thermal and radiation stability. The presence of heat generating isotopes (e.g., .sup.37 Cs and .sup.90 Sr) necessitates that the exchange medium be stable towards radiation had high temperatures.
The term "molecular sieve" refers to a wide variety of positive ion containing crystalline materials of both natural and synthetic varieties which exhibit the property of acting as sieves on a molecular scale. A major class of molecular sieves are the zeolites, although other crystalline materials are included in the broad definition. Examples of such other crystalline materials include coal, special active carbons, porous glass, microporous beryllium oxide powders, and layer silicates modified by exchange with organic cations. See, D. W. Breck, "Zeolite Molecular Sieves: Structure, Chemistry, and Use", John Wiley & Sons, 1974.
Zeolites are crystalline, hydrated, framework aluminosilicates which are based on a three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygens. Zeolites may be represented by the empirical formula EQU M.sub.2/n).A1.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
wherein, x is generally equal to or greater than 2 since AiO.sub.4 tetrahedra are joined only to Sio.sub.4 tetrahedra, and n is the cation valence. The framework contains channels and interconnected voids which are occupied by the cation, M, and water molecules. The cations may be mobile and exchangeable to varying degrees by other cations. Intracrystalline zeolitic water in many zeolites is removed continuously and reversibly. In many other zeolites, mineral and synthetic, cation exchange or dehydration may produce structural changes in the framework. Ammonium and alkylammonium cations may be incorporated in synthetic zeolites, e.g., NH.sub.4, Ch.sub.3 NH.sub.3, (CH.sub.3).sub.2 NH.sub.2, (CH.sub.3).sub.3 NH, and (CH.sub.3).sub.4 N. In some synthetic zeolites, aluminum cations may be substituted by gallium ions and silicon ions by germanium or phosphorus ions. The latter necessitates a modification of the structural formula.
The structural formula of a zeolite is best expressed for the crystallographic unit cell as: M.sub.x/n [(AlO.sub.2).sub.33 (SiO.sub.2).sub.y ].wH.sub.2 O where M is the cation of valence n, w is the number of water molecules and the ratio y/x usually has values of 1-100 depending upon the structure. The sum (x+y) is the total number of tetrahedra in the unit cell. The complex within the [] represents the framework composition.
The zeolites described in the patent literature and published journals are designated by letters or other convenient symbols. Exemplary of these materials are Zeolite A (U.S. Pat. No. 2,882,243), Zeolite X (U.S. Pat. No. 2,882,244), Zeolite Y (U.S. Pat. No. 3,130,007), Zeolite ZSM-5 (U.S. Pat. No. 3,702,886), Zeolite ZSM-11 (U.S. Pat. No. 3,709,979), and Zeolite ZSM-12 (U.S. Pat. No. 3,832,449).
Although there are 34 species of zeolite minerals and over 150 types of synthetic zeolites, only a few have been found to have practical significance. Many of the zeolites, after dehydration, are permeated by very small channel systems which are not interpenetrating and which may contain serious diffusion blocks. In other cases dehydration irreversibly disturbs the framework structure and the positions of metal cations, so that the structure partially collapses and dehydration is not completely reversible. Zeolites generally have only limited stability in acid and thus the use of such zeolites in removing radioactive wastes from acidic nuclear waste streams is precluded.
There has been considerable interest in developing metallosilicates other than zeolites which exhibit molecular sieve characteristics. For example, U.S. Pat. Nos. 3,329,480 and 3,329,481 disclose crystalline zircano-silicates and titano-silicates, respectively. U.S. Pat. No. 3,329,384 discloses Group IVB metallosilicates. U.S. Pat. Nos. 4,208,305, 4,238,315 and 4,337,176 disclose iron silicates. U.S. Pat. No. 4,329,328 discloses zinco--, stanno--, and titano-silicates. European patent application Nos. 0 038 682 and 0 044 740 disclose cobalt silicates. European patent application No.0 050 525 discloses nickel silicate.
U.S. Pat. Nos. 3,769,386, 4,192,778 and 4,339,354 disclose rare earth metal containing silicates. U.S. Pat. No. 3,769,386 discloses zeolitic alumino-metallosilicates crystallized from an aqueous reaction mixture containing Na.sub.2 O, SiO.sub.2, Al.sub.2 O.sub.3 and R.sub.2/n wherein R is Mg, Ca, Y, Fe, Co, Ni or a rare earth metal and n is the valence of R. U.S. Pat. No. 4,192,778 discloses rare earth exchanged zeolites of the faujasite type in which the equivalent of Na is less than 0.1 and the rare earth is at least 0.9 equivalent per gram atom of aluminum. U.S. Pat. No. 4,339,354 discloses a catalyst comprising a crystalline aluminosilicate such as zeolite Y, an inorganic matrix, and discrete particles of alumina, the catalyst having specified alkali metal and rare earth metal contents.
U.S. Pat. No. 4,486,397 discloses metallophosphate molecular sieves represented in terms of mole ratios of oxides by the formula EQU x(M'.sub.2 O):y(M.sub.2 O.sub.3):z(P.sub.2 O.sub.5):nH.sub.2 O
wherein M' is an alkali metal, M is a Group IIIB metal, x/y is a number ranging from about 1.1 to about 1.9, z/y is a number ranging from about 1.1 to about 1.9, and n/y is a number ranging from zero to about 8. This patent, which issued to, among others, one of co-inventors (Michael J. Desmond) of the invention disclosed and claimed herein, indicates that these metallophosphate molecular sieves are useful as ion-exchange materials.